In recent years, there has been a rapid advancement in realizing portable and cordless electronic apparatuses such as audio-visual apparatuses, personal computers and portable communication equipment. Conventionally, aqueous type batteries such as nickel cadmium batteries and nickel-metal-hydride batteries have been used as power sources for driving these electronic apparatuses. In recent years, however, non-aqueous electrolyte batteries represented by lithium ion secondary batteries that can be charged rapidly and have both high volume energy density and high weight energy density have become a mainstream of batteries used as such power sources. On the other hand, the nickel cadmium batteries and the nickel-metal-hydride batteries mentioned above are used as power sources for driving cordless power tools and electric vehicles that require heavy load characteristics, and higher capacity and higher large-current discharge characteristics are needed.
An electrode plate for the above-mentioned types of batteries is usually manufactured by applying an electrode active material in a slurry form onto a current collector made of a metal foil or a porous metal plate having a lengthy belt shape and drying the same to form an active material layer. The current collector on which an active material layer is formed (hereinafter, a current collector on which an active material layer is formed is referred to as an electrode plate precursor) is rolled with rollers, for example, to have a prescribed thickness, and then subjected to a slit processing to have a prescribed width and cut into a prescribed length to complete an electrode plate for a battery.
Herein, as shown in FIGS. 12 to 14, there are several embodiments of an electrode plate precursor which is a current collector on which an active material layer is formed. In FIG. 12, an active material layer 32 is formed by applying uniformly an active material onto a current collector 31. In FIG. 13, by applying intermittently an active material onto a current collector 31, a plurality of active material applied portions 32A are aligned with a prescribed pitch in the longitudinal direction of an electrode plate precursor (current collector 31) with active material unapplied portions 33 sandwiched between each thereof, thereby constituting an active material layer 32 (so-called intermittent application). In FIG. 14, by applying an active material in a stripe form onto a current collector 31 by dividing the same in the width direction, strips of applied portions 32B are aligned in the width direction of an electrode plate precursor (current collector 31) to form an active material layer 32 (so-called stripe application).
In any of these embodiments, active material unapplied portions 35 are formed on both sides in the width direction of the electrode plate precursor. Since these unapplied portions 35 are portions that are cut off at the time of cutting the electrode plate precursor to manufacture an electrode for a battery, the smaller the width thereof is made, the more the material loss can be reduced. Therefore, it is preferable to minimize the width of the unapplied portions 35. However, even when the width of the unapplied portions 35 is minimized, if the flatness of the active material layer 32 or that of the active material applied portions 32A and 32B, in particular the flatness in the width direction of the electrode plate precursor is not ensured, it is necessary to cut off both end portions in the width direction of the electrode plate precursor including both end portions of the active material layer 32 and the like for a reason described later. In consequence, the material loss cannot be reduced in case the flatness of the active material layer 32 and the like is not ensured.
It is to be noted that an active material unapplied portion is formed on both sides in the width direction of the electrode plate precursor because when a paste mainly composed of an active material is applied while a current collector in a lengthy belt shape is guided to the longitudinal direction, there is a limit in the precision of the application position because the current collector sometimes meanders slightly. Also, there is a possibility that the paste after the application bulges out to the width direction because of sag (state where the application form of the paste cannot be maintained because of low viscosity or low thixotropy) and the like.
Then, these years, in the above-described rolling process, the density of the applied active material is further increased by increasing pressure force in order to give a battery a higher capacity. This rolling deforms the electrode plate precursor. Herein, there is no problem as long as the deformation of the electrode plate precursor in the rolling process is such that the decrease in the thickness is due to uniform extension along the surface direction and is balanced. Otherwise, various problems and quality defects may be caused.
For example, there arise problems such as a “bend” in which an electrode plate precursor after the rolling has a protrusion on one of its surfaces, and a “crease” in which irregular roughness is produced on the current collector of an electrode plate precursor after the rolling. Problems of bends or creases lead to difficulties in winding up the electrode plate precursor in a coil form.
Herein, the reason why the electrode plate precursor is not extended uniformly along the surface direction is that the active material applied portions and unapplied portions are present on the electrode plate precursor. For example, in the case where the rolling is performed by guiding the electrode plate precursor in a belt shape to the longitudinal direction so as to pass it through a pair of rollers, pressure force is applied to only the active material applied portions and hardly any pressure force is applied to unapplied portions on both sides in the width direction of the electrode plate precursor. As in this case, when there is a difference in pressure force applied to the electrode plate precursor between the active material applied portions and unapplied portions, a difference in extension between the two is produced, and this difference in extension may produce creases or cuts in the boundary between the applied portions and the unapplied portions.
Also, in the case where deformation caused by the rolling is only due to deformation along the surface direction of the electrode plate precursor, if the deformation is not uniform between both sides in the width direction, a “warpage” in which the electrode plate precursor after the rolling bends to right and left is produced. When such a warpage is produced, a “winding displacement” in which the electrode plate is displaced to the axis direction of a core member is caused at the time of constituting an electrode plate group by winding in a spiral form an electrode plate for a battery produced through the above-described slit processing etc. Further, in the case where the binding force of the active material applied onto the current collector cannot catch up with extension of the current collector by the rolling, a “crack” is formed on the surface of the active material layer. An electrode plate for a battery in which a crease or a crack is formed on the electrode plate precursor readily causes separation of the active material, and manufacture of batteries using such an electrode plate for a battery may result in severe quality defects, particularly in the case of manufacturing lithium ion secondary batteries.
Herein, the application of the active material onto the current collector is generally carried out by using a die (see e.g. Patent Documents 1 and 2). The die comprises a manifold (a paste storing part) for storing a paste supplied by a paste supplying means, and a slit (a flat discharge flow path) for discharging the paste from the manifold (see FIG. 1 of Patent Document 1). Also, when the paste comprising the active material is applied onto the current collector by using the die, it is necessary to block so that the paste does not bulge out to the width direction of the current collector. For this reason, as shown in FIG. 15, both edge portions in the width direction of the active material layer 32 may be raised depending on the viscosity and the thixotropy of the paste. In this case, stress is concentrated onto these portions at the time of the rolling, which may result in cuts. For this reason, as described in Patent Document 3, not only the active material unapplied portions but also both end portions of the applied portions are cut off, and in this case, a material loss of an expensive active material is caused.
On the other hand, in the case where a paste of an active material having a high fluidity is applied at the time of applying the active material, a cross section of the active material layer in the width direction after the application often has such a shape that the thickness is decreased as approaching both ends, as shown in FIG. 16. In the case where an electrode plate for a battery is produced by rolling the electrode plate precursor having such a shape and then performing a slit processing into a prescribed width, an electrode plate for a battery cut out from both end portions readily causes a warpage. Further, since the active material layer 32 of the electrode plate for a battery cut out from both end portions has a smaller thickness than that of an electrode plate for a battery cut out from the central portion of the electrode plate precursor, in case a battery is produced by using the electrode plate for a battery cut out from both end portions, the battery has a smaller capacity. From this viewpoint, an electrode plate for a battery cut out from both end portions of an electrode plate precursor where both end portions in the width direction of the active material layer 32 are raised has the active material layer 32 having a larger thickness, and a battery using this has a larger battery capacity.
In consequence, in the case where flatness in the width direction of the active material layer 32 on the electrode plate precursor is not ensured, the electrode plate for a battery cut out from both end sides of the electrode plate precursor cannot be used for producing a battery and is thrown away. As a result, the material loss is increased.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2001-293414    Patent Document 2: Japanese Laid-Open Patent Publication No. 2003-187788    Patent Document 3: Japanese Laid-Open Patent Publication No. Hei 11-176424    Patent Document 4: Japanese Laid-Open Patent Publication No. 2003-145007